Metal halide lamps for general lighting are efficient and produce high quality white light. However, such lamps frequently require substantial time after ignition to warm up to nominal light output and steady state operation. Indeed, such lamps can require as much as several minutes from ignition to reach full light output, depending on the lamp type.
Metal halide lamps may be run-up more quickly to full light output by temporarily overpowering the lamp during the run-up process. While the temporary application of high current is not necessarily a problem, it can lead to thermal shock, electrode damage, and wall blackening. This is because conventional metal halide lamps are designed to operate with a relatively high steady state voltage and a relatively low steady state current at a nominal power P, where P=I*V. As a result, the electrodes in conventional metal halide lamps are not appropriately sized or otherwise configured to conduct the high current applied while the lamp is overpowered, leading to reduced lamp lifetime, lumen maintenance, etc.
The above issues are exemplified in various automotive (quartz) metal halide lamps. In such lamps, high current is applied during run-up to increase deposited power, which causes the lamp temperature to quickly rise. As the lamp temperature rises, the metal halide fill begins to evaporate, further increasing lamp voltage and deposited power. Although this run-up process allows automotive metal halide lamps to run-up relatively quickly, such lamps are not rated for long life, and do not permit the use of certain fills for higher quality light. Accordingly, such lamps are not ideal for general lighting applications, where rapid run-up, long lamp lifetime, and high quality photometric output are desired.